1. Field of the Invention
The present invention relates to the field of signal transmission and reception systems using a receiver which employs the superheterodyne system, and in particular, to systems which are adapted to share the radio frequency portion by using the transmission oscillator of the transmitter as the local oscillator of the receiver.
2. Description of the Prior Art
Conventional communication systems, typically radio or wireless telephones, of the duplex transmission system having a receiver of the superheterodyne system are designed such that the output of the transmission oscillator (channel oscillator) is modulated by an aural signal, etc., the oscillator output thus modulated is transmitted from an antenna at a transmission frequency fT through an amplifier and an antenna duplexer, a receiving signal of the frequency fR fed to a receiver through the antenna and the antenna duplexer is mixed at a mixer, with a signal fT generated by a local oscillator of the receiver, and an intermediate frequency fI which is lower than the frequency fR of the receiving signal is extracted through an intermediate frequency filter and picked up as an aural signal by a frequency discriminator.
In this case, to simplify the construction of the antenna duplexer, the system is usually designed so as to satisfy .vertline.fT-fR.vertline.=fI. In such design, since the oscillation frequency of the local oscillator is the same fT with that of the transmission oscillator, a single oscillator may be shared for both the local and the transmission oscillators, whereby cost is reduced.
However, the above method involves a problem that when a single oscillator is used for the above two oscillators, the transmission signal modulated by the aural signal, etc. is directly demodulated by the receiver so that it can be unfavorably heard as a side tone.
Solutions to the above problem are disclosed in U.S. patent application No. 337,609 filed Jan. 7, 1982 and Japanese patent application No. 54-68469. However, since the system of the U.S. patent application is designed to cancel the transmitter input contained in the demodulation signal by the addition or subtraction of the transmitter input after the demodulation of the receiving signal, addition/subtraction control is complex. On the other hand, the system of the Japanese Patent Application has a drawback that since the construction is such that the output of a first oscillator modulated by a modulation signal and the output of a second oscillator are mixed and the mixed output is fed to a second mixer, the modulation signal component cannot be removed sufficiently.